In an electronic key system of a vehicle, a transmitter in a vehicle transmits an interrogatory signal toward a predetermined detection area at every fixed time interval. When a vehicle user having a portable electronic key comes in the detection area, the electronic key transmits its identification (ID) code signal in response to the interrogatory signal. If the ID code agrees to an ID code assigned to the vehicle, a door lock control system in the vehicle drives its doors to an unlocking standby condition. The doors are unlocked electronically when a touch sensor detects that the user touches or manipulates a door handle.
The electronic key has a built-in battery to be operable to receive the interrogatory signal and transmit the ID code signal. The electronic key also has a mechanical key so that the vehicle doors may be unlocked by the mechanical key and door key cylinders when the electronic key becomes inoperable due to rundown of its built-in battery. Further, an immobilizer composed of a coil antenna and electronic circuits is provided in the vehicle to enable starting of the engine with the mechanical key by retrieving the ID code from the electronic key. The coil antenna is incorporated in an engine key cylinder of the vehicle so that the coil antenna generates an induction voltage to transmit its ID code signal. Thus, the electronic key system needs various backup systems such as the mechanical key, the key cylinders and the immobilizer, complicating a system structure.